The present invention is generally directed to the production of Group III metal nitride materials for use as free-standing articles as well as substrates for further processes and/or microelectronic and optoelectronic devices. In particular, the present invention is directed to the production of low-defect density single-crystal materials grown from a strain-relieving layer of single-crystal columns, utilizing enhanced sputtering techniques.
A wide variety of techniques exist for depositing thin films onto substrates in order to achieve desirable properties which are either different from, similar to, or superior to the properties of the substrates themselves. Thin films are employed in many kinds of optical, electrical, magnetic, chemical, mechanical and thermal applications. Optical applications include reflective/anti-reflective coatings, interference filters, memory storage in compact disc form, and waveguides. Electrical applications include insulating, conducting and semiconductor devices, as well as piezoelectric drivers. Magnetic applications include memory discs. Chemical applications include barriers to diffusion or alloying (e.g., galling), protection against oxidation or corrosion, and gas or liquid sensors. Mechanical applications include tribological (wear-resistant) coatings, materials having desirable hardness or adhesion properties, and micromechanics. Thermal applications include barrier layers and heat sinks.
Bulk materials can be used as substrates upon which microelectronic and optical devices are fabricated. Wide bandgap semiconductor materials such as gallium nitride, aluminum nitride, indium nitride and their alloys are being studied for their potential application in microelectronics and opto-electronics. These materials are particularly well suited for short wavelength optical applications, such as green, blue and UV light emitting devices (LEDs and LDs), and visible and solar-blind UV detectors. The use of UV or blue GaN-based LEDs makes possible the fabrication of solid state white light sources, with higher efficiencies and lifetimes 10 to 100 times longer than conventional sources. Additionally, GaN has a region of negative differential mobility with a high peak electron velocity and high-saturated velocity, which can be used for fabricating high-speed switching and microwave components. P-type doping of GaN and AlGaN with relatively high hole concentrations is now readily achieved, and ohmic and Schottky contacts have been characterized for n- and p-type materials. Thus, many of the above devices have or potentially have large, technologically important markets. Such markets include display technology, optical storage technology, and space-based communications and detection systems. Other applications include high temperature microelectronics, opto-electronic devices, piezoelectric and acousto-optic modulators, negative-electron affinity devices and radiation/EMP hard devices for military and space uses.
Attempts to grow low-defect density gallium nitride (GaN) thin films heteroepitaxially on substrates such as sapphire and silicon carbide (SiC) have had limited success. GaN materials heteroepitaxially grown on these substrates suffer from large concentrations of threading defects, typically on the order of 10xe2x88x928-10xe2x88x9210 cmxe2x88x922, due to the large lattice mismatch between the film and substrate. Threading defects increase leakage currents in diode and FET structures and act as a significant source of noise in photodetectors. As a result, the operation of high performance devices, such as high-speed, high-sensitivity UV photodetectors, and high power, high frequency microelectronic devices, is presently limited. Buffer layers of AlN, GaN, and other materials have been used to reduce the lattice mismatch. However, threading defects and low angle grain boundaries remain in the films. Differences between the film and substrate thermal expansion coefficients also result in stresses in the films.
Accordingly, homoepitaxial growth of GaN thin films on bulk GaN substrates is of great interest. The use of GaN substrates would eliminate the problems due to lattice mismatch and thermal expansion mismatch. Unfortunately, the availability of GaN substrates has been limited due to conventional processing capabilities. This problem has hindered the development of devices based on GaN and related nitride semiconductors. Several obstacles exist to the successful manufacturing and commercializing of high device-quality Group III nitride-based materials, whether in bulk, single-crystal, polycrystalline or epitaxial form, for electronics and other applications. These obstacles generally include cost, reproducibility, and purity.
For instance, gallium nitride has a high equilibrium vapor pressure of nitrogen that results in its decomposition at elevated temperatures. The solubility of nitrogen in gallium metal at room temperature and pressure is very low. As a result, conventional crystal growth methods to produce GaN are not practical. This has led to the development of several alternate bulk growth methods, including high-temperature, high-pressure (15 kbar) solution growth, evaporation, and sublimation.
Currently, aluminum nitride and gallium nitride exist only as polycrystalline or powder forms, or in thin films. Polycrystalline bulk aluminum nitride can be manufactured using powder processing techniques. This process has not yielded semiconductor-grade single crystal material. Formidable problems are associated with such techniques, beginning with the production of pure aluminum nitride powders and then the sintering of oxygen-free and defect-free aluminum nitride. Some of these problems include the production of both high-purity and uniform particle-size powders. The highest purity powders can contain up to 1% of oxygen and binders, such as Y2O3, that are needed to produce aluminum nitride with a high density. Therefore, high density is achievable at the expense of contamination. Sintering of these aluminum nitride powders is also a difficult process. The covalent nature of aluminum nitride prevents densification of pure aluminum nitride at low temperatures. Aluminum nitride decomposes at high temperatures, such as above 1600xc2x0 C., thereby preventing densification. Hence, costly sintering aids such as high pressures and impurities are required for producing high-density material. Other problems associated with powder processing of aluminum nitride include maintaining the purity and integrity of the powder, controlling the environment at high sintering temperatures, and the production of defect-free parts. Aluminum nitride is very difficult to manufacture using powder processing techniques without introducing contamination that will have adverse effects on the optical and thermal properties of the material. These impurities can be present in the crystalline lattice structure, and can migrate to the grain boundaries during sintering, causing the infrared absorbance to be high.
Various masking techniques have been explored in conjunction with lateral epitaxial overgrowth (LEO) and selective area growth (SAG) techniques in search of improved methods for fabricating low-defect density gallium nitride crystal layers. For example, U.S. Pat. No. 6,153,010 discloses a method for growing nitride semiconductor crystals. A nitride buffer layer is first grown on a substrate using gaseous Group III element and nitrogen sources (e.g., MOVPE, MBE, or HVPE). Using a vapor-phase technique and photolithography, an oxide selective growth mask is formed on the underlayer. The mask is configured as discrete stripes so that areas of the buffer layer remain exposed. Nitride semiconductor material portions are then grown on these exposed areas using gaseous Group III element and nitrogen sources. When such growth exceeds the upper ends of the mask stripes, the semiconductor material grows laterally on the mask stripes. Continued vertical growth results in the material portions combining to form an integral nitride semiconductor crystal. This process can be repeated to grow a second integral nitride semiconductor crystal from a second selective growth mask formed on the first integral nitride semiconductor crystal. In another disclosed embodiment, a nitride semiconductor layer is deposited on the substrate/buffer layer, and recesses are etched into the semiconductor layer. A first growth control mask is formed on the remaining top surfaces of the semiconductor layer, and a second growth control mask is formed in the respective bottom surfaces of the recesses. Nitride semiconductor material portions are then grown in the recesses by a vapor-phase technique that relies on gaseous Group III element and nitrogen sources. This is followed by lateral growth and the formation of an integral nitride semiconductor crystal.
In view of the state of the art as described hereinabove, it would be advantageous to provide a method and apparatus for growing device-quality nitride material without the need for masking and etching procedures.
As disclosed hereinbelow, it has now been discovered that enhanced sputtering techniques, which are physical vapor deposition (PVD) techniques, can be feasibly utilized to produce low-defect density Group III metal nitride materials of bulk thickness and of device-quality crystal, and thus utilized as part of methods of the invention disclosed hereinbelow. Magnetron sputtering is traditionally associated with thin film deposition. An advantage of sputter synthesis is that high purity compounds can be formed directly from the high purity source materials. Moreover, the synthesis can be achieved under highly controlled conditions. Nitrogen and Group III metals such as aluminum are readily available, from multiple sources, in ultra-high purity grades (e.g., 99.9999%) for the microelectronics industry. Sputter synthesis is currently the process that most effectively eliminates hydrogen from the bulk, since the sputter environment is controllable to ultra-high vacuum conditions. Through sputter synthesis of Group III nitrides, it is possible to obtain materials that have properties near the bulk properties. Since this takes place under ultra-high vacuum conditions, hydrogen and oxygen can be eliminated from the material. Reactive sputtering has the advantage of producing high purity materials with high densities, and ease of fabrication of quality crystalline material.
However, traditional magnetron sputtering has several drawbacks, which has made it very difficult to produce bulk materials. These drawbacks include unwanted target reactions, transport limitations, and low growth rates. During reactive magnetron sputtering, micro-arcs can occur on the cathode surface which cause imperfections in the deposited material. Another problem associated with this process is the xe2x80x9cdisappearing anodexe2x80x9d effect, in which the entire anode becomes covered by randomly grown insulating layers. Also related to this process is the problematic formation of an insulating nitride layer on the target surface that increases the impedance of the cathode until the target becomes xe2x80x9cpoisonedxe2x80x9d or completely insulating. This results in a drastic decrease in deposition rates to almost zero when the target becomes too nitrided to operate. Materials transport can also be a problem in bulk crystal growth using magnetron sputtering since there can be a significant loss of material to the sidewalls.
The present invention is provided to address these and other problems associated with the growth of thin films and bulk materials.
The present invention provides a method that utilizes sputter transport techniques to produce arrays or layers of self-forming, self-oriented columnar structures characterized as discrete, single-crystal Group III nitride posts or columns on various substrates. This columnar structure is formed in a single growth step, and therefore does not require processing steps for depositing, patterning, and etching growth masks. Characterization of such structures reveals discrete Group III nitride columns exhibiting a hexagonal crystal habit. Pursuant to the invention, the columns can be grown several tens of microns thick while maintaining the average column size. In some embodiments of the invention, the columns are advantageously employed as a starting structure for the growth of continuous, low defect-density, bulk materials. In a process termed herein as columnar epitaxial overgrowth (CEO), the highly oriented columnar structure is grown on a substrate or template material and, subsequently, the growth conditions are changed to effect coalescence of the Group III nitride material at the tops of the columns. As a result, a bulk crystal is grown on the columnar base structure in a single growth run. The bulk crystal has a distinct advantage over the conventional growth of layers directly on a substrate, in that the thermal mismatch stress between the bulk crystal and the substrate is mitigated by the intervening presence of the column structure. This, in turn, greatly reduces the degree of cracking and bowing in the resulting multi-layered structure, and eases post-growth processing. The crystal quality of the bulk crystal grown on the columns is characterized as being superior to or at least equivalent to Group III nitride layers grown directly on a substrate such as sapphire.
According to one method of the present invention, single-crystal MIIIN columns are produced. A template material having an epitaxial-initiating growth surface is provided. A Group III metal target in a plasma-enhanced reaction chamber is sputtered to produce a Group III metal source vapor. A nitrogen-containing gas is introduced into the reaction chamber. The III/V ratio of Group III metal source vapor to nitrogen is adjusted to create a Group III metal-rich environment within the reaction chamber conducive to preferential column growth. The Group III metal source vapor is combined with the nitrogen-containing gas to produce a reactant vapor species comprising Group III metal and nitrogen. The reactant vapor species are deposited on the growth surface to produce single-crystal MIIIN columns thereon.
The growth temperature is within a range of approximately 400xc2x0 C. to approximately 1200xc2x0 C. and, more preferably, within a range of approximately 600xc2x0 C. to approximately 1000xc2x0 C.
In one aspect of the method, the reactant vapor species are deposited directly on the growth surface. In an alternative aspect, an intermediate layer is deposited on the growth surface prior to depositing the reactant vapor species. Preferably, the columns have an average thickness that is greater than that of the intermediate layer. Additionally, the columns have an defect density lower than that of the intermediate layer and the template material.
Preferably, the columns grown according to the invention have an average thickness of approximately 0.5 micron or greater, and a defect density of 108 defects per cm2 or less.
According to another method of the invention, the III/V ratio is readjusted to create an environment within the reaction chamber that is conducive to columnar epitaxial overgrowth, such that continued deposition of the reactant species on the growing MIIIN columns results in the upper regions of the columns coalescing so as to form a substantially continuous, single-crystal MIIIN layer. Preferably, this MIIIN layer has a lower defect density than that of the columns.
According to yet another method of the invention, MIIIN layer on the columns is used as a buffer or transition layer for the growth of a bulk, single-crystal homoepitaxial or heteroepitaxial MIIIN article.
A broad range of techniques can be employed for growing any intermediate layer or MIIIN article according to the methods of the invention, including physical vapor deposition, sputtering, molecular beam epitaxy, atmospheric chemical vapor deposition, low pressure chemical vapor deposition, plasma-enhanced chemical vapor deposition, metallorganic chemical vapor deposition, evaporation, sublimation, and hydride vapor phase epitaxy.
According to one embodiment of the invention, a single-crystal column is produced according to methods described herein. Preferably, the column has a height of approximately 0.5 micron or greater, a lateral dimension of approximately 0.05 micron or greater, and a defect density of approximately 108 defects per cm2 or less.
According to yet another method of the invention, a single-crystal MIIIN article is produced. A template material having an epitaxial-initiating growth surface is provided. A Group III metal target is sputtered in a plasma-enhanced reaction chamber to produce a Group III metal source vapor. A nitrogen-containing gas is introduced into the reaction chamber. The III/V ratio of Group III metal source vapor to nitrogen-containing gas is adjusted to create a Group III metal-rich environment within the reaction chamber conducive to preferential column growth. The Group III metal source vapor is combined with the nitrogen-containing gas to produce a reactant vapor species comprising Group III metal and nitrogen. The reactant vapor species is deposited on the growth surface to produce single-crystal MIIIN columns thereon. A bulk, single-crystal MIIIN article is grown on the MIIIN columns.
Preferably, the MIIIN article has a thickness ranging from approximately 1 micron to greater than 1 mm, and a diameter ranging from approximately 0.5 inch to approximately 12 inches.
According to still another method of the invention, the MIIIN article is released to provide a free-standing, single-crystal MIIIN article. The removal technique utilized can be, for example, of polishing, chemomechanical polishing, laser-induced liftoff, cleaving, wet etching, and dry etching.
According to a further method of the invention, a wafer is cut from the MIIIN article. A surface of the wafer can be prepared for epitaxial growth, such as by polishing, and an epitaxial layer can then be grown on the prepared surface.
According to a still further method of the invention, a device or component is fabricated on the MIIIN article.
According to another embodiment of the invention, a bulk, single-crystal MIIIN article is produced by one of the above-described methods. Preferably, the MIIIN article has a diameter ranging from approximately 0.5 inch to approximately 12 inches and a thickness of approximately 50 microns or greater.
According to yet another embodiment of the invention, the article is produced in the form of a wafer having a thickness ranging from approximately 50 microns to approximately 1 mm.
According to still another embodiment of the invention, the article is produced in the form of a boule having a diameter of approximately 2 inches or greater and a thickness ranging from approximately 1 mm to greater than approximately 100 mm.
Methods of the present invention can be implemented by providing a novel sputter material transport device to enhance thin-film and bulk material manufacturing processes. The novel transport device is capable of ultra-high deposition and growth rates, making it feasible for growing thick material and increasing throughput in manufacturing processes. The transport device can be used both to grow bulk crystalline materials and to deposit thin films and epitaxial layers onto bulk substrates. Generally, as compared to other sputter processes, the transport device of the present invention has the advantages of lowered processing pressure, higher deposition rates, higher ionization efficiency, and a controlled processing environment with no contamination. The transport device utilizes an enhanced sputtering process to rapidly deposit both metallic and dielectric materials. This enhancement allows the process to overcome the limitations of conventional PVD techniques.
The transport device according to the present invention can achieve growth rates in excess of ten times those achieved by any other direct deposition process. As currently tested, the device is capable of depositing single or polycrystalline material at a rate in excess of approximately 60 xcexcm/hr. This high deposition rate allows for high throughput capabilities and the possibility of manufacturing bulk materials in short time periods. The device has increased growth rates due to the very high ionization efficiencies, which enhances the sputtering process without xe2x80x9cpoisoningxe2x80x9d the sputtering material. The ability to deposit material at high deposition rates will have many commercial applications, including high-throughput manufacturing processes of thick films of exotic materials. Moreover, high-quality material can be deposited in a cost-effective manner. It is also projected that the device will aid in the commercialization of bulk dielectric and semiconductor materials and will have numerous applications to other materials.
The transport device according to the invention surpasses present technology by offering a non-contaminating method, as implemented by a triode sputtering device, to increase the ionization efficiency and hence the overall deposition rate. The device also has the advantage of a cooler operating temperature than a thermionic hollow cathode configuration, allowing the injector means of the device to be composed of low-temperature materials, and thus can apply to a broad range of materials as compared to conventional processes. The transport device can increase the deposition rate of the target material and lower the sputtering pressure, thereby enabling a line-of-sight deposition process.
The transport device is capable of growing bulk material such as aluminum nitride and other Group III nitrides and also is capable of depositing metal in deep trenches for the semiconductor industry.
According to the present invention, the transport device includes a magnetron source and a non-thermionic electron (or, in effect, a plasma) injector assembly to enhance magnetron plasma. Preferably, the electron/plasma injector assembly is disposed just below the surface of a cathode target material, and includes a plurality of non-thermionic, hollow cathode-type injector devices for injecting electrons into a magnetic field produced by a magnetron source. The injector can be scaled in a variety of configurations (e.g., circular or linear) to accommodate various magnetron shapes. When provided in the form of a circular ring, the injector includes multiple hollow cathodes located around the inner diameter of the ring.
The transport device constitutes an improvement over previously developed hollow cathode enhanced magnetron sputtering devices, in that the device of the present invention is a non-thermionic electron emitter operating as a xe2x80x9ccoldxe2x80x9d plasma source and can be composed of the same material as its sputtering target. The injector can be manufactured out of high-purity metals (e.g., 99.9999%), thereby eliminating a source of contamination in the growing film. The addition of the injector to the magnetron sputtering process allows higher deposition rates as compared to rates previously achieved by conventional magnetron sputtering devices. Moreover, the transport device takes advantage of the hollow cathode effect by injecting electrons and plasma into the magnetic field to increase plasma densities without the contamination problem associated with a traditional, thermionic-emitting tantalum tip. As disclosed above, the transport device is further characterized by a decreased operating pressure and an increased ionization rate over conventional magnetron sputtering.
Therefore, according to another method of the present invention, single-crystal MIIIN columns are produced by using a sputtering apparatus comprising a non-thermionic assembly disposed in a reaction chamber to produce a Group III metal source vapor electron/plasma injector from a Group III metal target. A nitrogen-containing gas is introduced into the reaction chamber. The III/V ratio of Group III metal source vapor to nitrogen is adjusted to create a Group III metal-rich environment within the reaction chamber that is conducive to preferential column growth. The Group III metal source vapor is reacted with a nitrogen-containing gas to produce a reactant vapor species comprising Group III metal and nitrogen. The reactant vapor species is deposited on the growth surface of the template material to produce single-crystal MIIIN columns thereon.
The sputter transport device comprises a sealable or evacuable, pressure controlled chamber defining an interior space, a target cathode disposed in the chamber, and a substrate holder disposed in the chamber and spaced at a distance from the target cathode. The target cathode is preferably bonded to a target cathode holder and negatively biased. A magnetron assembly is disposed in the chamber proximate to the target cathode. A negatively-biased, non-thermionic electron/plasma injector assembly is disposed between the target cathode and the substrate holder. The injector assembly fluidly communicates with a reactive gas source and includes a plurality of hollow cathode-type structures. Each hollow cathode includes an orifice communicating with the interior space of the chamber.
According to one aspect of the present invention, the electron/plasma injector assembly is adapted for non-thermionically supplying plasma to a reaction chamber. The injector assembly comprises a main body and a plurality of replaceable or interchangeable gas nozzles. The main body has a generally annular orientation with respect to a central axis, and includes a process gas section and a cooling section. The process gas section defines a process gas chamber and the cooling section defines a heat transfer fluid reservoir. The gas nozzles are removably disposed in the main body in a radial orientation with respect to the central axis and in heat transferring relation to the heat transfer fluid reservoir. Each gas nozzle provides fluid communication between the process gas chamber and the exterior of the main body.
Methods of the invention entailing the use of the non-thermionic electron/plasma injector assembly can be utilized to grow a bulk, single-crystal MIIIN article on the MIIIN columns. The MIIIN article can be released to provide a free-standing article. In conjunction with any of the methods of the present invention, microelectronic or optoelectronic devices and/or components can be fabricated on the MIIIN layers or articles, or on any additional layer grown on the MIIIN layers or articles.
It is therefore an object of the present invention to provide a method for fabricating single-crystal Group III nitride layers or arrays of columns characterized by, among other advantageous properties, low defect density and high degree of orientation.
It is another object of the present invention to provide such columnar structures as strain-relieving buffer or transition layers or seed crystals for the growth of additional low-defect density Group III materials thereon.
It is yet another object of the present invention to provide a novel sputter material transport method and device capable of ultra-high deposition and growth rates of low-defect density, strain-relieving Group III nitride column structures and layers or articles on such structures.
Some of the objects of the invention having been stated hereinabove, other objects will become evident as the description proceeds when taken in connection with the accompanying drawings as best described hereinbelow.